Electrical machines can be categorized as motors or generators. The construction of electrical motors and generators is similar with many electrical machines being able to operate as both a motor and a generator.
Permanent magnet electrical machines have a winding that moves relative to the magnets. There are many different configurations of permanent magnet electrical machines. The winding may be part of the stator of the machine with the magnets attached to the rotor, or the winding may be part of the rotor with the magnets attached to the stator. Permanent magnet electrical machines may be brushed or brushless.
The geometry of an electrical machine can be categorized by the direction of the flux that the winding is exposed to. Axial flux machines have a disc shaped winding exposed to a magnetic flux that flows substantially parallel to the axis of rotation of the machine. The axial flux may be provided by two disc shaped sets of magnets with the winding positioned in the gap between the magnet sets. One of the sets of magnets may be replaced with iron only, to provide a return path for the magnetic field. In some applications there may be only one set of magnets without any iron on the other side of the winding. Radial flux machines have a cylindrically shaped winding exposed to a radially directed magnetic flux, which may be provided by two cylindrical sets of magnets, one cylindrical set of magnets and an iron cylinder to provide a return path for the magnetic field, or one cylindrical set of magnets alone. Similarly, conical flux machines have a conically shaped winding with magnets providing flux normal to the surface of the conical shaped winding. Linear electrical machines have a flat rectangular shaped winding that moves linearly relative to one or two rectangular sets of magnets that provide flux normal to the rectangular shaped winding.
The winding of a traditional electrical machine has conductors wound onto a laminated iron core. The iron core is primarily used to reduce the reluctance of the magnetic circuit, but it serves a secondary purpose of providing a supporting structure and thermal path for heat generated in the winding.
Ironless machines have windings without any ferromagnetic material. When optimised, an ironless machine can have extremely low spinning losses, no cogging torque, high peak torque and minimal torque ripple. Ironless machines find applications in servo drives, hard disk drives and have significant potential for use in vehicle drives or other applications that require low spinning losses and high transient torque. With the introduction of high energy rare earth permanent magnets, the ability to produce high performance ironless machines became a reality.
In order to obtain the maximum performance from an ironless machine, it is necessary to maximise the amount of conducting material, typically copper, in the winding. This is referred to as the “fill factor”. An electric machine has an active volume that is dedicated to be filled by the active portions of the winding conductors. In an axial flux ironless machine with two sets of magnets, this active volume is the volume between the faces of the two sets of magnets. The fill factor is defined as the proportion of the active volume taken up by conducting material.
In ironless machines, the conductors in the winding are exposed to the full magnetic field and so to avoid excessive eddy currents in the winding the conductors are typically made from fine strands of insulated copper wire. However, reducing the copper wire diameter to reduce eddy currents also makes it difficult to manufacture a winding with a high fill factor because of the thickness of the insulation on the wire and the gaps between the wire strands. Machines with poor fill factor require large magnetic content to achieve acceptable performance, and machines with poor fill factor typically suffer from poor thermal conductivity due to the excessive quantity of resin required to construct the winding.
“Litz wire” is a conductor made from a bundle or bundles of strands of fine copper wire. The wire strands have a thin film of insulation, and the entire bundle is surrounded by an insulating layer made from textile yarn, tape or other materials. Litz wire has several configurations, known as “types”. Type-1 Litz wire is comprised of a single bundle of strands of copper wire twisted together within the outer insulating layer. Type-2 Litz wire is comprised of multiple sub-bundles of strands of copper wire. The strands of each sub-bundle are twisted together, like Type-1 Litz wire, and the sub-bundles themselves are then twisted together within the outer insulating layer. Type-2 Litz wire does not have any additional insulating layers surrounding the sub-bundles. Type-3 Litz wire is also comprised of multiple sub-bundles of strands of copper wire twisted together within the outer insulating layer but unlike Type-2 Litz wire, each sub-bundle has its own outer insulating layer. The sub-bundles of Type-3 Litz wire may comprise a single twist of strands, like Type-1 Litz wire, or multiple twists of strands like Type-2 Litz wire. As supplied, Litz wire is flexible.
Windings for ironless machines using conductors made from Type-1 or Type-2 Litz wire are known, but use of Type-3 Litz wire is not known. The paper titled “Design of an in-wheel motor for a solar-powered electric vehicle” by V. S. Ramsden, B. C. Mecrow and H. C. Lovatt, published in the IEE Proceedings, “Electric Power Applications”, 1998, 145, No 5, 402-408, discloses an ironless direct drive wheel motor with a winding made from Type-2 Litz wire. International Patent Publication WO 2004/010561 A1 (Hamilton Sundstrand) also discloses the use of Litz wire to make a winding for an electric machine, and shows an example using Type-2 Litz wire.
Many methods of manufacturing windings for ironless machines have been proposed or used. U.S. Pat. No. 5,744,896 (Kessinger et al) discloses a method of manufacturing a winding for an ironless machine in which the winding is made up of a large number of individually wound coils of wire strands. The coils are assembled in a mould and wired together. The disadvantages of this method are that each coil must be individually electrically connected and small variations between each coil affect the overall performance of the machine.
An alternative to individually wound coils is to arrange the conductors of the winding into a wave pattern. The conductors of a wave pattern winding are wave shaped, meaning that each conductor progresses around the winding, periodically crossing the magnetic field, without looping back on itself. Each wave shaped conductor has a plurality of legs that are the active part of the conductor directly exposed to the magnetic field and end turns that connect the legs. It is desirable to minimise the length of the end turns in order to minimise resistance losses in the machine, and to minimise the weight and size of the winding. Compact end turns can be difficult to achieve in a multiphase wave pattern winding because the end turns overlap each other.
Some wave pattern windings have two layers, with the legs of each conductor alternating between the two layers. U.S. Pat. No. 5,319,844 (Huang et al) discloses a radial air gap motor having a two layer wave pattern winding. However, this motor has flux concentrators positioned in the gaps between the legs of the wave shaped conductors, rather than being a purely ironless motor.
The winding of the motor disclosed in the above referenced paper “Design of an in-wheel motor for a solar-powered electric vehicle” is manufactured by arranging Litz wire into a two layer wave pattern then bonding the Litz wire with resin in a mould to form a rigid structure. The wave pattern is formed by winding the Litz wire directly in the mould, or winding the Litz wire onto a temporary former, such as a board with pegs, then transferring the wave pattern into the mould. Whilst this method of manufacturing a winding is well suited to light weight solar-powered vehicles, a winding having a higher copper fill factor than this prior art method provides is desirable for higher power applications.
U.S. Pat. No. 6,649,844 (Kusumtoto et al) discloses a method of preforming a wave shaped conductor from a bundle of strands of copper wire. The end turns of the wave shaped conductors disclosed in this patent are straight and lie in the plane of the wave shape, resulting in a square wave shaped conductor. These square wave shaped conductors are arranged into a wave pattern with large gaps between adjacent conductor legs. This method of forming a wave pattern winding is directed to electric machines having an iron core to fill the large gaps between the conductor legs of the winding. However, this method is not well suited to manufacturing ironless wave pattern windings because the square wave shaped conductors result in an inefficient packing of the end turns once the gap between the legs of the conductors is reduced, as is necessary for an efficient ironless winding arrangement.
It is an object of the present invention to provide an improved winding arrangement for an electric machine.